1. Field of the Invention
The present invention relates to an optical polymer composition, and more particularly, to an optical polymer composition for use in an optical waveguide.
2. Description of the Related Art
Some polymers have been used for an optical substrate and an optical lens or a compact disk. Recently, research has been conducted on polymers to be used as a material for an optical waveguide for light of near infrared wavelength.
The polymers which have been applied to this field, however, absorb the light of 1,000.about.1,700 nm which is in the near infrared. Such absorption of near infrared rays by the polymer is caused by overtone of harmonics of stretching and deformation vibrations of chemical bonds between carbon and hydrogen (C--H bond) in alkyl, phenyl or other similar functional groups. Thus, using a polymer having these groups as a material for an optical waveguide utilizing light in the near infrared wavelength range results in a large optical loss. In order to reduce this optical loss, the light absorption wavelength of the polymer should be shifted to the wavelength region which is longer or shorter than the wavelength of the near infrared rays. To this end, the substitution of fluoride (F) or heavy hydrogen (deuterium, D) for the hydrogen (H) of the C--H bond has been suggested.
In case of substituting deuterium for the hydrogen, carbon-deuterium (C--D) bond absorbs a large amount of light of 1,500 nm, so that this method is not suitable to form a material to be used in an optical communications device utilizing light of 1500 nm wavelength. It is known, though, that the method for substituting F for H minimizes the optical absorption loss in the wavelength range of 1,000.about.1,700 nm.
When fabricating an optical waveguide device, a core is required to be surrounded by a cladding, such that an optical signal is guided through the core. Here, the cladding should have a refractive index lower than that of the core within the wavelength region for optical communications. This difference in refractive indices between the core and the cladding determines the size of an optical waveguide and affects the design of an optical waveguide device.
The refractive index of a polymer is in general determined by its molecular structure. In a conventional method for preparing materials for a core and a cladding each having a different refractive index, a first polymer having a predetermined refractive index is obtained, and the first polymer is then partially modified to attain a second polymer whose refractive index is slightly different from that of the first polymer. Such prepared first and second polymers are used as materials for the core and the cladding, respectively.
The material for an optical waveguide device must satisfy the following conditions. First, considering the optical characteristic aspect, optical absorption by the material itself must be low in order to reduce the optical loss. Considering the process characteristic aspect, film forming properties should be suitable to fabricate an optical waveguide. Also, the material must have thermal and polarizing characteristics so as to resist damages caused by external factors during the manufacturing process.
Separately synthesizing a polymer having a predetermined refractive index as well as satisfying the above requirements, however, requires much labor and time, and to achieve such synthesis is very difficult in practice. The refractive index of the core should be greater than that of the cladding as described above. Thus, when a polymer for the cladding is prepared to have a similar structure to that of a polymer for the core, with different amounts of fluorine substitution, the amount of F contained in the polymer for the core should be reduced compared to the polymer for the cladding. Such a reduction in the amount of F in the polymer, however, requires the presence of C--H bonds, and causes optical loss by the light absorption of the polymer.
One suggested method to attain materials for the core and cladding which satisfy a desired difference in the refractive index, is a method for mixing two compounds having different refractive indices in a predetermined mixing ratio. However, this method causes phase separation.
Based on our observation of the art, we have found that what is needed is an optical polymer composition suitable for use in a near infrared optical waveguide. The polymer must have low optical loss, allow for adjustment of refractive index, and not lead to phase separation of mixed polymers.